The thymic hormone thymopoietin (TP) has been shown to play a regulatory role in immune, nervous, and endocrine functions and has been isolated from bovine and human thymus. For additional general information on TP, see, also, G. H. Sunshine et al, J. Immunol., 120:1594-1599 (1978); G. Goldstein, Nature, 247:11-14 (1974); D. H. Schlesinger and G. Goldstein, Cell, 5:361-365 (1975); G. Goldstein et al., Lancet 2:256-262 (1975). TP has also been found to be present in brain extracts [R. H. Brown, et al., Brain Research 381:237-243 (1986)].
Prior art bioassays which attempted to measure TP have been reported to be cumbersome, inaccurate and unreliable [J. J. Twomey, et al., Proc. Natl. Acad. Sci. USA, 74:2541-2545 (1977); V. M. Lewis, et al., J. Clin. Endo. Metab. 47.:145-150 (1978); J. J. Twomey, et al., Am. J. Med. 68:377-380 (1980)]. Immunoassays are the preferred format for measuring peptides and proteins in plasma or serum, but prior attempts to develop immunoassays to measure TP have not yielded clinically useful techniques. For example, a displacement radioimmunoassay (RIA) for measuring bovine TP was developed that detected TP concentrations greater than 5 ng/mL in tissue extracts. However this RIA is incapable of measuring TP levels in serum [see, e.g., G. Goldstein, J. Immunol. 117:690-692 (1976)].
The sensitivity of the TP RIA was subsequently increased to 20 picograms (pg) [see, e.g., P. J. Lisi et al, Clin. Chim. ACTA, 107:111-119 (1980)] using "human serum-based standards" and rabbit antisera. However, this assay has not proved effective or reproducible in practice. In addition, the present inventors observed that 20 picogram sensitivity is too poor to detect human blood levels of TP.
A sandwich enzyme-linked immunoassay (ELISA) was later developed for bovine TP using a combination of polyclonal and monoclonal antibodies [A. Fuccello et al, Arch. Biochem. Biophys., 228:292-298 (1984)]. Although the assay provided specificity in distinguishing bovine TP from bovine splenin, it proved ineffective in measuring serum TP in humans.
There is considerable interest in the medical community in measuring TP levels for several reasons. For example, it has been found that as the thymus involutes with age, thymic hormone levels decrease, which is believed to be related to increased susceptibility to disease in aging [G. Goldstein and I. R. Mackay, The Human Thymus, Wm. Heineman Med. Books Ltd., London (1969)]. Additionally, hypersecretion of TP has been implicated in myasthenia gravis [G. Goldstein, Lancet, 2:1164-1167 (1966)], as being involved in the impairment of transmission of signal from nerve to muscle. When this signal is interrupted, the result is generalized weakness.
Thus, an assay capable of accurately detecting levels of human thymopoietin in plasma and serum is not yet available.
In view of the clinical significance for TP, and the lack of current methods capable of accurately detecting total levels of these factors in both plasma and serum, there is a significant unmet need in the art for a reliable method of accurately measuring total levels of specific peptide factors in human plasma and serum.